1. Field of the Invention
The present invention relates to an angular velocity sensor interface circuit and an angular velocity detection apparatus used to compensate for camera shake.
2. Description of the Related Art
In general, a digital still camera and a digital video camera include angular velocity sensors to detect and compensate for camera shake.
Such an angular velocity sensor used to compensate for camera shake includes a piezoelectric vibrator. A voltage is generated by vibration of the piezoelectric vibrator caused by a Coriolis force and outputs a voltage signal generated in accordance with an angular velocity. Note that the voltage signal output from the angular velocity sensor is amplified and converted into a voltage signal generated in accordance with an angular velocity in a control operation corresponding to an angular velocity, such as camera shake compensation, using an output signal of the angular velocity sensor. A DC amplifier circuit is provided to amplify the voltage signal output from the angular velocity sensor and inevitably generates a direct-current offset due to variations in circuit constants. Furthermore, angular velocity changes in frequencies below a predetermined frequency are not caused by camera shake, but rather, are caused when a user intentionally performs specific camera operations, such a spanning and tilting. Therefore, such frequencies below the predetermined frequency should be ignored when detecting camera shake. As disclosed in Japanese Unexamined Patent Application Publication No. 5-207356, a DC component and a low-frequency component of a signal output from an angular velocity sensor are removed using a high-pass filter and then the output signal is subjected to a DC amplifying processing.
An angular velocity sensor interface circuit disclosed in Japanese Unexamined Patent Application Publication No. 5-207356 will now be described with reference to FIG. 1.
In FIG. 1, the angular velocity sensor interface circuit includes a high-pass filter 12 and a DC amplifier circuit. An angular velocity sensor 11 receives a power supply voltage VCC and outputs an angular velocity detection signal Vo generated in accordance with an angular velocity. The high-pass filter 12 includes a capacitor C1 and a resistor R1. The DC amplifier circuit includes resistors R2 and R3 and an operational amplifier OP.
The high-pass filter 12 further includes a switch SW which causes a short circuit at both terminals of the resistor R1. The switch SW is turned on when the angular velocity sensor 11 and the angular velocity sensor interface circuit are activated and causes a short circuit at both terminals of the resistor R1. With this configuration, a time constant of the capacitor C1 included in the high-pass filter 12 at the charge time is minimal and the capacitor C1 is charged at high speed up to a steady-state voltage so that a signal output from the angular velocity sensor interface circuit can be immediately utilized.
The capacitor C1 included in the high-pass filter 12 shown in FIG. 1 has a relatively large capacitance, such as several tens of μF. Even when the resistor R1 is short-circuited using the switch SW shown in FIG. 1, it takes a few seconds to charge the capacitor C1 up to the steady-state voltage. Accordingly, even if the angular velocity detection apparatus is provided to compensate for camera shake, for example, the camera shake compensation cannot be performed simultaneously upon power-on of a camera.
FIG. 2 is a diagram showing waveforms output from various sections of the angular velocity sensor interface circuit shown in FIG. 1. When a voltage Va at a point A, which is the angular velocity detection signal Vo obtained by applying the power supply voltage VCC, is greater than a reference voltage Vref, that is, when the voltage Va has a positive offset, an output voltage VaO of the operational amplifier OP is first clamped by the power supply voltage VCC, then decreases as a voltage Vb at a point B decreases, and eventually is restored to the reference voltage Vref.
Since a digital video camera in the related art requires a waiting time including a time for loading a tape and a required time due to various resource constraints during a time immediately after the digital video camera is powered on and before shooting, even when a response speed of an angular velocity detection apparatus is relatively slow, this does not cause a problem. However, in recent years, digital still cameras and DVD video cameras capable of shooting immediately after being powered on, that is, digital still cameras and DVD video cameras having high-speed responses, have been required.